1. Field of the Invention
The present invention relates to an ink jet recording method for recording a multi-color image by using the inks of a plurality of colors and a recording head having a plurality of nozzles corresponding to such colors, and particularly to an ink jet recording method of using a recording head in which the number of nozzles for a specific color is greater than the number of nozzles for other colors.
2. Related Background Art
Conventionally, methods of recording color image using four colors of yellow (Y), magenta (M), cyan (C) and black (Bk) have been well known and applied to printers or copying machines.
As one technique for such methods, an ink jet recording method of using four ink jet recording heads corresponding to four colors to permit independent discharging of four colors has been put to practical use. This method has the merits that the recording heads can be used in common when the amount of discharging the ink for each color is equal, and the driving method of each recording head can be simple, because the recording heads are independent for each color.
On the other hand, this method has the drawback that it is unsuitable for the smaller apparatus in the recent trend, since four recording heads are mounted, resulting in higher costs and complicated assembling.
Also, since the four recording heads are used, this method is suitable for the higher speed, but if the printing is made on the recording sheet on which the ink is less liable to fix, such as a plain paper, the blur of color inks, may easily occur, resulting in the image quality remarkably degraded.
Therefore, a recording head has been proposed in which nozzle (discharge unit) groups for discharging the inks of four colors of yellow, magenta, cyan and black are arranged not to be overlapped in a scan direction, as shown in FIG. 1. This recording head allows the use of single recording head to provide a multi-color image, and is very suitable for inexpensive and small-sized apparatuses. Also, this recording head hardly produces the blur of inks, resulting in higher quality of image, although it takes longer time to perform the printing, as compared with the use of the four recording heads.
However, the use of this recording head in which the nozzle groups for four colors are not overlapped, as shown in FIG. 1, can not completely prevent the blur of color inks from occurring. Also, it is naturally required that the color printers or color copying machines produce the black image of high quality, comparable to that of conventional white and black apparatuses. To meet that requirement, an attempt has been made to make the discharge amount of black ink greater than that of color inks. However, in this case, there was a problem that the blur of inks was remarkable at the boundaries between black image and color image, resulting in the quite bad print quality.
Also, various proposals for recording the color image using plural color inks have been made to achieve the higher quality recording and higher speed recording.
In Japanese Laid-Open Patent Application No. 58-138656 (hereinafter referred to as conventional example 1), the present applicant has proposed a method of arranging an increased number of ink jet nozzles corresponding to only a main color K among CMYK to attain a recording speed in the recording mode of only color K faster than in the color recording mode.
Also, the present applicant has proposed a method for improving the image quality in Japanese Laid-Open Patent Application No. 58-173669 (hereinafter referred to as conventional example 2) and Japanese Laid-Open Patent Application No. 3-146355 (hereinafter referred to as conventional example 3). According to conventional example 3, a proposal has been made to achieve the overall improvement by discharging a greater amount of K ink than that of other inks. Also, according to this conventional example 3, another proposal has been made to prevent the blur of K in such a manner that other colors than K are not recorded around the K.
Also, according to U.S. Pat. No. 4,967,203 (hereinafter referred to as conventional example 4), a proposal has been made to suppress the blur as a whole in such a manner as to record all colors in either a cross pattern or a counter-cross pattern at the forward recording.
However, in the above method of conventional example 1, the number of nozzles used for K is equal to the number of nozzles for CMY at the color recording, which means that no improvement in the recording speed or recording quality is expected at the color recording.
Also, in the above proposal of conventional example 2, the overall image quality may be enhanced, but in some cases, the blur of K may spread over CMY at the color recording, resulting in degraded image quality.
Also, in the above proposal of conventional example 3, unrecorded pixels around K may expose the underlying color, for example, white on the white paper, resulting in degraded image quality.
Also, in the above proposal of conventional example 4, the substantial recording may slow down to one-half thereof. The same recording speed can be attained by doubling the number of nozzles for the recording head, but there is the disadvantage of increasing the costs of an expensive recording head or a control or drive circuit section.
Suppose now an instance wherein an ink jet head (recording head) has discharge nozzle groups for yellow (Y), magenta (M), cyan (C) and black (Bk), and the number of black nozzles is greater than the number of nozzles for each color, or wherein the longitudinal length of black nozzle group is equal to or greater than the longitudinal length of font data, or the longitudinal length of color nozzle groups is equal to or greater than the longitudinal length of font data. The color recording with such recording head is accomplished by using all the nozzles of the heads for yellow, magenta and cyan, and only predetermined nozzles of black head, while reciprocating the recording head in a main scan direction by multiple times. Also, the recording of monochrome image is performed by discharging the ink onto the recording sheet, using all the nozzles of black head.
Therefore, conventionally, in performing the recording with such recording head, when control is transferred from the color recording to the monochrome recording, the recording operation of using all the nozzles including other nozzles than the predetermined nozzles of black head as described above which are not used during the color recording is started. However, the other nozzles which are not used before may not be fully activated, and therefore it is apprehended that the discharged ink causes an unevenness at the initiation of the recording. Such black unevenness may arise not only when switching is made from the color recording to the monochrome recording, but also when font data with its width exceeding the longitudinal length of nozzle group for the recording head is recorded in monochrome, because different section of the nozzles of the black head is used by each scan of the recording head, causing a problem that the recorded image has degraded quality.